Method of producing molded resin product

ABSTRACT

A molded resin product can be produced without shrinkage on the surface by providing a hollow part and foam cells therein. Before molten resin containing a foaming gas is introduced into a cavity between molds, the cavity is preliminarily filled with a gas with pressure high enough to prevent the foaming gas from beginning to foam or expand. While the molten resin is injected into the cavity or after its injection is over, another high-pressure gas is injected through at least one gas injection nozzles into the resin where a hollow part is desired. As the nozzle is retracted from the cavity, the high-pressure gas is removed from the cavity, allowing the foaming gas to begin to foam and expand, forming a hollow part and foam cells inside the molded product.

This is a division of application Ser. No. 08/594,818, filed Jan. 31,1996, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a method of producing molded resin productshaving thick-wall parts.

According to the so-called counter-pressure gas method, which is anexample of injection molding method for producing a resin product havingthick-wall parts, a gas (such as air or nitrogen gas) with pressureincreased beyond the atmospheric pressure is preliminarily injected intoa cavity between mold pieces before molten resin is injected thereinto.When a molten resin material having an organic solvent (such asalcohol), an inorganic liquid (such as water), an organic gas such as(Cl₃ H), an inorganic gas (such as N₂, CO₂ and CO) or their mixturedissolved therein as a foaming agent is subsequently injected into thecavity, the foaming gas is thereby prevented from beginning to foam orexpand. Only after the preliminarily injected high-pressure gas isremoved from the cavity and the pressure inside the cavity issufficiently reduced, the foaming gas begins to foam and expand insidethe product being molded, thereby preventing shrinkage on its surfaces.

The so-called gas assisted injection method is another molding methodwhereby a cavity in a mold is fully or partially filled with a resinmaterial by primary injection and thereafter, or while it is beinginjected, a gas is injected into the product being molded.

With the counter-pressure gas method, as described above, the occurrenceof shrinkage is intended to be prevented by the force of foaming insidethe produce being molded. Thus, the shrinkage force in skin layers onthe surfaces cannot be overcome by the force of foaming unless thefoaming layer is sufficiently thick (as shown in FIG. 6). In otherwords, products to be formed by this method are under a severelimitation regarding their shape and wall thickness. In order to obtaina molded product with a smooth surface and a foam layer inside, the wallthickness indicated by arrows in FIG. 7 must be over 5-6 mm.

By the gas assisted injection method, by contrast, it is intended toprevent the occurrence of shrinkage by the pressure of the injected gas.Without the use of any foaming agent, thick-wall parts of a product arenot cooled quickly and the high-pressure gas does not effectively formany cavity inside the product being molded, as shown in FIG. 8. Instead,the gas is spread throughout the molded product, affecting adversely thestrength of the product. In the case of a product having a thick rib ona thin-wall part, such as shown in FIG. 9, the thin-wall part coolsquickly and the gas cannot diffuse to such a part in time, making ahollow cavity only in the rib part. Accordingly, the thickness of thewall parts should be at least about 5 mm.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to eliminate the problems ofprior art technology described above.

It is more specifically an object of this invention to a method ofproducing an injection-molded product having both thick-wall andthin-wall parts and fine foam cells and hollow parts formed in thethick-wall part such that occurrence of surface defects such asshrinkage is prevented.

An injection-molded product to be produced according to this invention,may be characterized as having hollow parts where shrinkage wouldotherwise be likely to occur such as its thick-wall parts, as well asfoam cells between its surface layers and these hollow parts and aroundthe hollow parts. Throughout herein, the expression "foam cells" will beused to indicate small bubbles formed inside a molded resin product bythe foaming of a forming gas during its production, and the expression"hollow parts" will be used to indicate larger cavities formed inside amolded resin product by injecting a high-pressure gas.

Before molten resin is poured into the cavity in an injection moldingapparatus to produce such a product, a foaming gas is already containedin the resin material and a gas with pressure greater than theatmospheric pressure is supplied into the cavity prior to the injectionof the resin material. During or after the injection of the resinmaterial into the cavity, another high-pressure gas at pressure greaterthan the atmospheric pressure is injected at selected positions insidethe product being molded, thereby creating hollow parts. Thesehigh-pressure gases are kept inside the cavity for a specified length oftime so as to prevent the foaming gas from beginning to foam or expandprematurely. As these high-pressure gases are discharged from the cavityas well as the interior of the product being molded, the foaming gascontained in the molten resin starts to foam and expand such that foamcells are formed between the surface layers and the hollow parts, aswell as around the hollow parts, of the product.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate an embodiment of the invention and,together with the description, serve to explain the principles of theinvention. In the drawings:

FIG. 1A is a drawing for showing gas flow routes within a moldingapparatus used according to this invention, and; FIG. 1B is a drawingfor showing a portion of FIG. 1B more in detail, FIGS. 1A and 1Btogether being sometimes referred to as FIG. 1;

FIG. 2 is a schematic sectional view of a gas injection nozzle;

FIG. 3 is a sectional view taken along line 3--3 of FIG. 2;

FIG. 4 is a diagonal external view of a product obtained by using theapparatus shown in FIGS. 1-3 according to this invention;

FIGS. 5A, 5B, 5C and 5D are sectional views of the product of FIG. 4taken along the line 5--5 in FIG. 4, FIG. 5A showing its outline, FIG.5B being a view immediately after the injection of the resin containingfoaming agent, FIG. 5C being a view after the injection of thehollowness-creating gas, and FIG. 5D being a view after thecounter-pressure and hollowness-creating gases have been discharged;

FIG. 6 is a sectional view of a shrinkage on the surface of a moldedproduct with a thick-wall part produced by a prior art method;

FIG. 7 is sectional view of a molded product with a thick-wall partwithout any shrinkage;

FIG. 8 is a sectional view of a generally thick molded product with ahollow part formed with the injection of a high-pressure gas;

FIG. 9 is a sectional view of a molded product with a thin-wall part anda hollow part by the injection of a high-pressure gas; and

FIGS. 10-1 and 10-2 illustrate a flow chart of the operation of themolding apparatus of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-3 will be referenced first to describe a molding apparatus usedaccording to this invention. As shown in FIG. 1, a cavity 2 in the formof a molded resin product to be obtained is provided between thecontacting surfaces of a mobile mold part 101 connected to a hydraulicactuator 111 and a stationary mold part 102 such that a product of thedesired shape can be obtained by injecting molten resin thereinto at ahigh pressure. An injection passageway 103 is provided at the center ofthe stationary mold part 102 such that the injection screw nozzle 3 of aknown type can be pushed forward at the time of a molding operation andbe connected to the opening to the passageway 103. The injection screwnozzle 3 contains therein a piston 301 which can be moved longitudinallyforward and backward by means of an actuator 304 and is provided with ahelically threaded peripheral surface 302 so as to be able to rotate bymeans of a motor (not shown). The mobile mold part 101 is provided withone or more gas injection nozzles 5 (only one being shown in FIGS. 1Aand 1B) which are for injecting a high-pressure gas (referred to as "thehollowness-creating gas"), corresponding to positions where surfaceshrinkage is likely to occur on the molded product, and can be advancedor retracted longitudinally by means of another hydraulic actuator 503,an ejector 4 adapted to be activated by still another hydraulic actuator401 to eject the molded product out of the mold parts, acounter-pressure gas route 104, a gas supply route 105 to the gasinjection nozzles 5, and a return route 106 from the gas injectionnozzles 5. An O-ring is inserted between the contact surfaces of themold parts 101 and 102, and there is a gap for passing only a gas fromthe counter-pressure gas route 104 to the cavity 2. Additionally, thereare provided several detecting means including a detector switch LS1 fordetecting the position of the mobile mold part 101 where it contacts thestationary mold part 102, another detector switch LS2 for detecting theposition of the injection screw nozzle 3 where it has reached theinjection passageway 103, a still another detection switch LS4 fordetecting the position of the piston 301 of the injection screw nozzle 3indicative of the end of the injection of resin, and temperature sensorS for measuring the temperature of the mold on the side of thestationary mold part 102.

Next, the system for supplying the hollowness-creating gas to the gassupply route 105 will be explained. A high-pressure gas container 11containing nitrogen gas (normally at pressure of about 150 kg/cm²) isconnected to a reserve tank 19 through a manually operable valve 12 anda flow route 18 containing a pressure gauge 13, a pressure control valve14, another pressure gauge 15, a safety valve 16 and a check valve 17.The reserve tank 19 is provided with a pressure gauge 21, a safety valve22 and a manually operable discharge valve 23 leading to a drain. Thenitrogen gas, which is temporarily stored in the reserve tank 19, issupplied through a check valve 26 and a flow route 27 into a diaphragmpump 28. After it is compressed thereby to a higher pressure (maximum500 kg/cm²), it is passed through a check valve 29 and a flow route 31and is stored in a high-pressure reserve tank 32.

The high-pressure reserve tank 32 is provided with a pressure gauge 33,a safety valve 34 and a manually operable valve 35 leading to a drain.The high-pressure gas stored in this high-pressure reserve tank 32passes through three parallel-connected circuits 36A, 36B and 36C forcontrolling pressure in three stages, being connected to the gas supplyroute 105 through a check valve 37 in a flow route 38. Each of theseparallel-connected circuits 36A, 36B and 36C includes, in order from theside of the high-pressure reserve tank 32, a check valve 41, a pressurecontrol valve 42, a pressure gauge 43 and an electromagnetic switchvalve 44 (or 44A, 44B and 44C, corresponding respectively to thecircuits 36A, 36B and 36C).

The return route 106 is connected to the reserve tank 19 through a route45, an automatic valve 46, and another return route 48 containing acheck valve 47.

Next, the system for sending a compressed gas such as air or an inactivegas with a pressure greater than the atmospheric pressure (referred toas "the counter-pressure gas") into the cavity 2 will be described.High-pressure air from an air compressor 51 (normally at a pressuregreater than the atmospheric pressure such as 30 kg/cm²) is introducedinto another reserve tank 55 through a circuit 54 including a filter 52and a check valve 53. The reserve tank 55 is provided with a pressuregauge 56, a safety valve 57 and a manually operable valve 58 leading toa drain and is connected to the counter-pressure gas route 104 through acheck valve 59 and a three-way electromagnetic valve 61.

A high-pressure nitrogen-gas container 62 is connected to anelectromagnetic three-way switch valve 61 for reducing the oxygenconcentration inside the cavity 2 through another gas flow route 69containing a manually operable valve 63, a pressure gauge 64, a pressurecontrol valve 65, another pressure gauge 66, a safety valve 67 and acheck valve 68. The high-pressure nitrogen gas container 62 may bereplaced by a device for generating nitrogen gas such as a liquidnitrogen evaporator provided with a booster or the like for maintainingthe gas pressure at a certain elevated level higher than that by the aircompressor 51. These devices may also be used as a combination.

More than one gas injection nozzles 5 may be provided, depending on theshape of the product to be molded, where surface shrinkage is likely tooccur and hence it is desired to form a hollow part. As shown more indetail in FIG. 2, each gas injection nozzle 5 is in the form of aneedle-containing throughhole 107 in the mobile mold part 101 with anopening on the contact surface abutting the cavity 2 at a positioncorresponding to a thick-wall portion of the product to be molded. Aneedle cylinder 502 with a conic front end part 502a is fastened to thehollow interior of a tubular outer cylinder 501 so as to be freelyslidable inside the throughhole 107, and a step 108 for providing asmaller opening diameter is formed and serves as a forward stopper forthe outer cylinder 501 of the needle cylinder 502. A detector switch LS3is provided for the contact of the outer cylinder 501 with the step 108.The outer cylinder 501 has a front end indentation 501a on its endsurface facing the cavity 2, and is fastened to the needle cylinder 502with a flange such that its conic front end part 502a protrudes forwardfrom its front end indentation 501a. The outer cylinder 501 is connectedto the forward end of a piston rod 504 of a hydraulic actuator 503 andis adapted to be retracted backward thereby.

The needle cylinder 502 has a center part with a space 502b having areduced inner diameter. Its front end is provided, as shown in FIG. 3,with four slits 502c on its outer circumference such that a pressuredgas can pass therethrough in the axial direction although molten resincannot. A radially extending flow route 501b is provided through theouter cylinder 501 such that, when the outer cylinder 501 is at its mostforwardly advanced position, this flow route 501b connects to the gassupply route 105. The return route 106 through the mobile mold part 101has an opening behind the step 108 but in front of the front end of theouter cylinder 501 when the latter is in the retracted position (shownin FIG. 2) so as to connect to the cavity 2. O-rings are provided wherethe outer cylinder 502 contacts the step 108 and slides against theinner wall of the throughhole 107 for preventing gas to flowthereacross.

A control unit for controlling the overall operation of the entireapparatus is schematically indicated at 80 in FIGS. 1A and 1B. Thecontrol unit 80 includes a driver circuit 81 for controlling theoperations of the various hydraulic actuators 111, 303, 304, 401 and503, a sequence control circuit 82 (to be described below more indetail) and timers T1-T6 for determining the timing at which variousvalves are to be operated. The timer T6 relates to the relationshipbetween the temperature of the stationary mold part 102 and the timingfor the removal of the hollowness-creating gas in order to cause thefoaming agent to start foaming. This relationship is experimentallydetermined and manually inputted (through an inputting means which isnot shown in FIG. 1) into a memory circuit 83. The mold temperaturemeasured by the temperature sensor S is compared by a comparing circuit84 with this relationship, and the timing for the timer T6 is setaccordingly by a timing-setting circuit 85. The memory circuit 83, thecomparing circuit 84 and the timing-setting circuit 85 may also beconsidered parts of the control unit 80.

Although only one gas injection nozzle 5 is shown in FIG. 1 for theconvenience of disclosure, one or more of such nozzles 5 are provided tothe mobile mold part 101 corresponding to a thick-wall part of theproduct to be molded. Pellets of a thermoplastic resin material, such asvinyl chloride, polycarbonate, styrene grated polyphenylene ether,polystyrene, acrylonitrile butadiene styrene (ABS) copolymer resin(ABS), high-impact polystyrene, styrene modified polyphenylene oxide andpolypropylene, are thrown inside a heating cylinder (not shown) togetherwith a foaming gas such as N₂ and a hydrocarbon gas, and a foaming agentsuch as inorganic agents like sodium bicarbonate, ammonium bicarbonateand sodium boron hydride and organic agents like azodicarbon amide(ADCA) to cause a physical or chemical reaction such that the pelletsare melted by the heat generated thereby. The resin density is increasedby applying a back pressure and the gas from the foaming agent issimultaneously caused to be dissolved.

The three parallel-connected circuits 36A, 36B and 36C may, for example,be adapted to provide a lower pressure, a medium pressure and a higherpressure. Since the time required for molten resin to start solidifyingdepends on the temperature of the mold parts 101 and 102 with which itcomes into contact, the functional relationship between the temperatureof the stationary mold part 102 as one would obtain by the temperaturesensor S and the timing of molten resin to solidify is preliminarilydetermined experimentally, and this experimentally obtained relationshipis stored in the memory circuit 83 through an input means (not shown).Depending upon the situation, two or all of the three pressure levels tobe provided by the three parallel-connected circuits 36A, 36B and 36Cmay be equal, and the relative magnitude of the pressure levels in thethree flow routes 36A, 36B and 36C may not necessarily be as describedabove.

Next, the basis operation of the apparatus will be explained withreference to the flow chart shown in FIG. 10. When the molten resin isready to be injected, the hydraulic actuator 111 is activated (S1) inresponse to a command from the driver circuit 81. When the detectorswitch LS1 detects that the two mold parts 101 and 102 have closed (YESin S2), the driver circuit 81 of the control unit 80 causes activatesthe hydraulic actuator 303 to thereby advance the screw nozzle 3 towardsthe passageway 103 (S3). If the detector switch LS2 detects that thescrew nozzle 3 has come into contact with the opening to the passageway103 (YES in S4), a compressed gas (with pressure greater than theatmospheric pressure) serving as the counter-pressure gas is sent intothe cavity 2 (S5) by activating the air compressor 51 to initially storecompressed air (with maximum pressure of 25 kg/cm², which is greaterthan the atmospheric pressure) inside the reserve tank 55, opening theelectromagnetic three-way switch valve 25 61 in response to a commandfrom the sequence control circuit 83 of the control unit 80 to connectthe routes 54 and 62 together such that the counter-pressure gas is sentinto the cavity 2 from the flow route 104 of the mold part 101 throughthe gap between the contact surfaces of the mold parts 101 and 102. Whenit is necessary to reduce the oxygen concentration inside the cavity 2(YES in S6), the manually operable valve 63 is opened by controlling thepressure of the inactive nitrogen gas from the high-pressure nitrogengas container 62 by the pressure control valve 65 to thereby introducethis nitrogen gas into the cavity 2 (S7). Instead of nitrogen, use maybe made of Ar, CO₂, CO and other non-combustible gases.

Next, actuator 304 is activated to advance the piston 301 of theinjection screw nozzle 3 in response to a command from the drivercircuit 81 of the control unit 80, thereby injecting molten resin with afoaming gas dissolved therein into the cavity 2 (S8). Either the resinis injected into the cavity 2 so as to completely fill it or an amountwhich is slightly less than the volume of the cavity 2 is injected. Thefoaming of the foaming gas, as well as its expansion, is controlled withthe help of the counter-pressure gas. The completion of the injection ofthe molten resin or the filling of the cavity 2 is detected, forexample, physically from the distance traveled by the injection screwnozzle 3 or electrically by means of the detector switch LS4 (YES inS9), and the timers T1 and T2 are started thereupon (S10 and S11).

The timer T1 controls the timing and thereby the operation of thehydraulic actuator 503 to move forward (towards the cavity 2) the outercylinder 501 of the needle cylinder 502 (S12) until the detector switchLS3 detects that it has reached its forwardly advanced position incontact with the step 108 serving as a stopper (YES in S13). Ahigh-pressure gas such as air, N₂, Ar or CO₂ at a pressure greater thanthe atmospheric pressure is injected (S14) through the gas injectionnozzles 5 (to serve as the hollowness-creating gas), while the N₂ gasfrom the gas container 11, with its pressure adjusted to 10-30 kg/cm² bythe pressure control valve 14, is transported through the flow route 18and temporarily stored in the reserve tank 19. Its pressure is thenfurther increased by the diaphragm pump 28 to a maximum value of 500kg/cm², and it is next stored in the high-pressure reserve tank 32.Three levels of pressure are prepared in the flow routes 36A, 36B and36C by means of their respective pressure control valves 42, dependingon the size and the shape of the product to be molded. For example, thefirst flow route 36A may be used to prepare a low gas pressure forinjecting the gas into the interior of the product being molded, thesecond flow route 36B may be for preparing a middle gas pressure forcausing the gas to expand inside the product, and the third flow route36C for preparing a high gas pressure for preventing the foaming gas inthe dissolved state from beginning to foam or expand.

The electromagnetic switch valve 44A of the first flow route 36A isopened first in response to a command from the control unit 80 and thetimer T3 is started (S15). When a preset time interval (also indicatedas T3), determined by the nature of the product being molded, has beencounted thereby, the electromagnetic switch valve 44B in the second flowroute 36B is opened by a command from the control unit 80 (S16) and thetimer T4 is started (S17). When another time interval T4, alsopredetermined according to the nature of the product being molded, hasbeen counted by the timer T4, the electromagnetic switch valve 44C inthe third flow route 36C is opened by a command from the control unit 80(S18) and the timer T5 is started (S19). As explained above, however,the relative pressure in the three flow routes 36A, 36B and 36C need notbe determined as in this example. For example, the pressure level in thethird flow route 36C may be between those in the first and second flowroutes 36A and 36B.

The compressed nitrogen gas, passing through the flow route 38 to serveas the hollowness-creating gas, is injected into the product beingmolded through the gas supply route 105 in the mobile mold part 101, theflow route 501b through the outer cylinder 501, the space 502b insidethe needle cylinder 502 and its four slits 502c. The surface of thethick-wall parts of the product being molded is cooled by the mold parts101 and 102 and quickly becomes solid, but the interior of the productsolidifies more slowly and is still in the molten condition when thecompressed gas is injected into this portion of the product being moldedso as to form a hollow part and to absorb the shrinkage that would occurby the cooling. The time (T5 set by the timer T5) required for theinjection of the compressed hollowness-creating gas depends on the shapeof the product being molded but it is usually on the order of severalseconds to several tens of seconds.

The counter-pressure gas, earlier introduced into the cavity 2 asdescribed above, is discharged therefrom by activating the three-wayswitch valve 61 to open the flow route 62 to the atmosphere (S22), inresponse to a command from the control unit 80, either before or afterthe injection of the resin material is completed (according to thetiming set by the timers T2 and T5 (YES in S20 and S21)). At the sametime, flow routes 69 and 54 are closed, the valve 44C is closed (S23)and the timer T6 is started (S24).

As explained above, the time interval T6 to be counted by the timer T6is determined (S30) by the comparing circuit 84 which compares thetemperature measured by the temperature sensor S (S29) and theinformation stored in the memory circuit 83 of the control unit 80. Whenthis time T6 is counted by the timer T6, the hydraulic actuator 503 isactivated to move the outer cylinder 501 (and hence also the needlecylinder 502) backwards (that is, away from the cavity 2) (S25) so as toopen the return path 106 to the interior of the throughhole 107, and theelectromagnetic switch valve 46 is activated at the same time such thatthe compressed gas in the cavity 2 is guided through the flow paths 108,45 and 48 into the reserve tank 19 and stored therein (S26).

With the counter-pressure gas and the hollowness-creating gas thusdischarged, the foaming gas, which has hitherto been prevented therebyfrom foaming and expanding begins to foam and expand to form a hollowpart and foam cells and to thereby counteract the volume shrinkage(S27). After another preset time period has been counted, the hydraulicactuators 111, 303 and 504 are activated by commands from the controlunit 80 to open the mold parts 101 and 102, and the actuator 401 isoperated to take out the molded product (S28). The discharge of thecounter-pressure gas may be effected either before, after orsimultaneously with the injection of the hollowness-creating gas.Similarly, the injection of the hollowness-creating gas need not beeffected after the injection of the resin material but may be startedwhile the resin material is being injected. The timing of the injectionmay be controlled, for example, by the same detector which detects theposition of the injection screw nozzle 3. These choices are to be made,depending on the shape of the product to be molded. If an inactive gasis used instead of air as the counter-pressure gas, it may be collectedafter discharge.

As an example, a product with the shape shown in FIGS. 4 and 5A wasmolded by using acrylonitrile butadien styrene copolymer resin and aninjection molding apparatus with capacity of 120 t. Molten resin with anaddition by 0.1% of azo-dicarbon amide (ADCA) as foaming agent was used.Nitrogen gas at pressure 18 kg/cm² was injected into the cavity 2 as thecounter-pressure gas. A hollowness-creating gas at pressure 50 kg/cm²was injected over a period of 5 seconds into the product being molded attwo positions. FIG. 5B shows the condition of the product after theresin material has been injected into the cavity. The counter-pressuregas was discharged simultaneously, and a hollowness-creating gas wasinjected to form hollow parts 91 and 92 as shown in, FIG. 5C. When thehollowness-creating gas was discharged 5 seconds later, one of thehollow parts (91) disappeared because the force of foaming was strongerthan the force associated with the shrinkage, there being left a foamlayer 96 as shown in FIG. 5D. Around the other of the hollow parts (92),however, the shrinkage force was sufficiently strong such that thehollow part 92 was left without collapsing. Thus, there was no shrinkagedetectable externally. The foaming multiplicity was about 5% and theratio of hollowness reached the value of 3%.

In summary, defects caused by shrinkage are successfully prevented fromappearing on the surface, especially of thick-wall parts of a moldedproduct, because the force associated with the shrinkage iscounter-acted by forming foam cells and hollow parts which serve toabsorb the force of shrinkage. Thus, molded products with smoothsurfaces can be obtained and the yield of high-quality molded productscan be improved. Since many foam cells, instead of hollow parts, areformed throughout the molded product, the overall strength of theproduct is also improved. Such products not only have improved heatinsulation and noise eliminating properties but also can provide bettercushioning effects and hence have better shock resistance.

Although the invention was described above with reference to only oneexample, this is not intended to limit the scope of the invention. Manymodification and variations are possible, for example, in the controlsystem of the apparatus by using different sets of detectors and timersto appropriately control the timing at which different flow routes ofthe gases are opened and closed. Thus, the control unit 80 is onlyschematically illustrated in FIG. 1 and its connection by dotted linesshould be only broadly interpreted.

What is claimed is:
 1. A method of producing a molded resin product byusing a injection molding apparatus having a pair of molds with a cavitytherebetween, said method comprising the steps of:preliminarily fillingsaid cavity with a counter-pressure gas with pressure greater than theatmospheric pressure, said counter-pressure gas comprising air;controlling the oxygen concentration of said counter-pressure gas bycontrollingly adding an inactive gas to said air; thereafter injectinginto said cavity molten resin containing a foaming gas; forming a hollowspace inside said molten resin by injecting a hollowness-creating gaswith pressure greater than the atmospheric pressure into said moltenresin at a specified position inside said product; and removing saidcounter-pressure gas and said hollowness-creating gas from said cavityand from within said product, thereby allowing said foaming gas to beginto foam and expand and causing foam cells to be formed in said product.2. The method of claim 1 wherein said hollowness-creating gas isinjected while said molten resin is being injected into said cavity. 3.The method of claim 1 wherein said hollowness-creating gas is injectedafter said molten resin has been injected into said cavity.
 4. Themethod of claim 1 further comprising the step of controlling thepressure of said-hollowness-creating gas in a plurality of levels.